Late Frost Fear: Protecting Fruit Trees from Losing the Entire Crop to Freeze Injury

Southwest Yard & Garden by Dr. Marisa Thompson

UPDATED 3-21-22 with some very uplifting news... scroll to the bottom!

Plum buds (left) are in “bud burst” and “first white” stages of floral development. Thirty minutes of exposure to temperatures in the mid-20s are expected to kill 10% of buds in these stages, and temperatures in the low 20s might kill as much as 90% of the flower buds. Peach buds (right) in this “first pink” stage are slightly more resilient than plum buds. Temperatures in the mid-20s are still expected to kill 10% of the buds, but in order to kill 90% temperatures would have to drop into the mid-teens. Photo credit J. van der Ploeg.

Question: How can I protect my backyard peach tree from losing fruit to late frosts?

-     Submitted via Patrick Kircher, Roosevelt County Extension Agriculture Agent, Portales, NM

Answer: This is a tough one. There’s lots of advice out there for things that might help (emphasis on the “might”). Your options for protecting those precious buds and flowers really depend so much on how old and big the tree is, what stage of development the buds are in, how cold it’s going to get, how high wind speeds get, and how long the cold lasts. Because eastern parts of the state might get a cold snap this week, I’m focusing on what to do to protect existing trees in your yard from late frosts. In another column, I explained the underlying principles in more detail and offer suggestions for how to select trees better suited for our challenging environment. Click here to link to that column!

There are some tried and true tricks for maximizing fruit load by protecting flower buds, open flowers, and developing fruit from freezing temperatures. Covering your tree in some way is likely the best option. This is just one reason selecting trees with dwarf rootstocks is recommended. If the tree is small enough, you can cover it with a pop-up tent like those used at farmers’ markets. Just trapping the warm air under that umbrella can keep temperatures more moderate underneath. (This is also a reason to pull back shade cloth or close big umbrellas at night in the summer to allow that trapped heat to escape.) Depending on how cold it’s going to get, adding an incandescent light bulb can bump the temperature up several degrees into the safety zone nearer freezing temperatures. Safety is key! Do not use heaters or anything with a flame that might catch fire. Depending on how high winds are expected to get, that canopy cover might not work because the warm air won’t be trapped as well.

If you expect high winds, don’t have a pop-up tent, or your tree won’t fit underneath it but it’s small enough to cover with a sheet or blanket, this is a good option. Wrapping the tree canopy without knocking off some of the flowers is impossible. The struggle is real. Ask a friend to help and take deep breaths. Avoid plastic tarp as a cover if it will be in direct contact with the buds because plastic isn’t a good insulator, and tissues touching the plastic may still get very cold. Whichever cover you use, be sure to uncover the tree again during the day.

Another idea is watering the root zone a day before the cold sets in so the ground below can hold more heat and keep that microclimate warmer. If there’s enough sun and enough time, painting a 5-gallon (or larger) water container black and allowing it to heat up in full sun during the day, then leaving it at the base of the tree where it can release heat slowly overnight can help. If there’s not enough time, use pre-heated water in the container. However, again, if temperatures drop very low or wind speeds get high, these techniques won’t work at keeping the warm air near the buds where you need it most. High winds over wet soil may even act like a swamp cooler and lower temperatures even further.

Some gardeners recommend stringing holiday lights around the canopy to help keep buds warm, and this might help, but not with LED lights. Buds that are near (but not touching) a bulb can benefit from the warmth. The trouble for me with this tip is that I feel certain I’d knock off all the flowers when stringing the lights.

When a friend in Clovis texted me for advice on March 2^nd on how to prepare his peach and plum trees for a cold front that was expected to bring temperatures down to 10°F with a wind chill closer to zero, I had more questions than answers. Are flower buds swelling or opening? How old are the trees?

He said they’re only one year in the ground and sent photos of buds on the plum just barely opening (aka “bud burst” and “first white” stages) and some buds on the peach tree in “first pink” stage of development.

Critical temperatures (in Fahrenheit) for 10% or 90% bud kill in peach and nectarine trees after 30 minutes exposure. From: https://extension.usu.edu/productionhort/fruit/tree/CriticalTemperaturesFrostDamageFruitTrees.pdf 

Similar resources: https://www.canr.msu.edu/uploads/files/PictureTableofFruitFreezeDamageThresholds.pdf?fbclid=IwAR3SeOLhhn_SqjqklNqGlFyUtadgZNAQMbMm_2-wrLRGfxL6bvXth5JGJNU

https://intermountainfruit.org/cold-effects/peach-nectarine-stages and here's another one for apricots: https://intermountainfruit.org/cold-effects/apricot-stages. 

With temperatures dropping that low and buds already opening to expose petals, I was afraid nothing would help. I contacted my predecessor, Dr. Curtis Smith, for his take. Dr. Smith warned that with the expected winds from that storm and trees that young, wrapping them with fabric might create a sail effect that could be strong enough to uproot the trees altogether. (Check out Dr. Smith's blog: SouthwestGardenSmith.)

The good news is that the early bud growth stages on those plum and peach trees were not as susceptible to freeze injury as they would have been at later stages of flowering or fruit development. Even though temperatures did get down to the teens in Clovis on March 4^th, photos of those trees from this week show two white flowers on the plum and quite a few pink flowers hanging on for dear life on the peach tree. Those flowers are likely from buds that weren’t yet swelling at the beginning of March. Generally, buds are hardier when they’re in an early development stage. That’s why dormant buds can last through the entire winter. Unfortunately, the tiny developing fruit are even more susceptible to being killed by frost after the petals fall.

Links to more information:

• Critical Spring Temperatures for Tree Fruit Bud Development Stages - Publication with photos by Michigan State Extension educator, Mark Longstroth, and data compiled from Washington and Michigan State Universities
• New Mexico State University Extension Publications:
• Fruits and Nuts for New Mexico Orchards Guide H-310
• Fruit Trees for the Home Orchard: Varieties and Management Guide CR-523
• Establishing Fruit and Shade Trees Guide H-420
• Shade Trees for New Mexico Guide H-426
• Why Fruit Trees Fail to Bear Guide H-308
• Link to ALL of NMSU's Extension publications sorted by topic: https://aces.nmsu.edu/pubs/howto/howto.html

UPDATED 3-21-22 with some very uplifting news:

Text from the Utah State infographic linked above: 

"The percentage bud kill which causes crop reduction will vary with each crop. For example, to have a full crop of cherries requires well over 50% bud survival in most years, while apples, pears, and peaches may only need 10-15% bud survival."

- Isn't that uplifting news!? --At least for apple-, pear-, and peach-growers!   You're welcome. ;)  -Marisa

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).

Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.

Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE